Water on the Knee refers to any long-term increase in the size of the knee joint. This may, in fact, be from the soft tissue or from fluid inside the knee joint. Swelling can be indicative of a more serious underlying condition of the knee, such as cartilage, tendon or ligament injury, arthritis, or internal bleeding. Swelling has many causes and it is important to have a specific diagnosis to guide specific treatment. Swelling of any type at the knee deserves evaluation by a knee expert.

As the name implies, the symptom or compliant to the doctor is most often that the knee is swollen. However, lesser amounts of swelling may not be noticed by the patient. In those cases, the symptoms may be a sensation of tightness or incomplete motion.